1. Field of the Invention
This invention relates to an apparatus for flattening sheets of material such as aluminum by pressing and baking.
2. Description of the Prior Art
There are a number of applications where it is desirous to have very flat sheets of material. For example, one means for magnetic recording is to rotate a disk of aluminum whose flat surfaces are coated with a magnetic film. Information is stored in the magnetic coating by various alignment of the magnetization. A recording transducer, positioned close to the rotating surface may either store information by radiating a magnetic field which realigns the magnetization, or it may retrieve information by detecting the alignment. The closer the transducer is to the moving surface, the denser can be the information stored on the disk. For this reason, the recording industry has endeavored to make disks flatter and flatter in keeping with the demand for greater and greater information storage density.
At the present time, requirements for flatness are in the range of 50-500 microinches over a five inch span.
According to the present state of the art, the steps taken to manufacture an aluminum disk substrate, include:
Stamping out a blank disk from a sheet of aluminum that is about 0.085" thick. (the outside diameter may range from 31/2 inches to 14 and the inside diameter may range from 1 to 6 inches depending on the ultimate use of the disk).
"Flatbaking" the disks which involves assembling the disks in vertical stacks, compressing the stack by laying heavy weights on top or enclosing the stack between platens which are forced together by springs, then placing the entire stack and compressing means into an oven, whence it is heated up to 650 degrees, F. and held at temperature for about two hours. At this temperature, the disks are sufficiently plastic that they flatten out due to the compressing force and retain their flat shape after cooling to room temperature.
The disk at this point is called a "blank" and is the form which is supplied to the recording media manufacturer for final machining, polishing and coating.
These latter steps include:
machining about 0.005 inches off each side of the disk. This operaation is performed to remove imperfections such as inclusions and scale in the disk surface which are introduced by the rolling process at the mill. Machining introduces "cold work" in the metal which causes the disks to warp. Therefore the manufacturer performs a second flatbake operation, identical to that described above in order to remove the warp. Then he polishes the disk so that it is now ready for coating.
The technique of flatbaking a second time as discussed above has been practiced for thirty years. There are a number of problems associated with this step which the present invention avoids. To understand the advantages of the invention, one must understand the differences in the crystalline state of the metal before the first flatbake and the second flatbake.
At the first flatbake stage, the crystals are small and strained by the rolling operation which has shaped the ingot into a sheet. A flatbake temperature of 650.degree. F. is required because at this temperature, grain growth can occur and reorientation can take place so that the disk is left in an annealed (soft) state.
However, at the second flatbake stage, the condition of the metal is that relatively minor imperfections have been introduced in the surface by machining-imperfections such as dislocations which are misalignments in the crystall lattice and can be readily removed at 450.degree. F. The presently used method of weights and spring does not flatten the disks at 450.degree. F. simply because too much force is required. In order to flatten the disks with this amount of pressure, it it necessary to go up to a temperature of 650.degree. F.
This involves expense in several ways.
When weights are used, the weights take up oven space and require heat; dwell time in the oven is considerably lengthened. When springs are used, the springs lose their temper at 650.degree. F. and eventually must be replaced. Dwell time in the oven--time to heat up to 650.degree. F., hold at temperature for two hours, cool to room temperature is typically 16 hours.
Another problem that results during the flat baking step is that, at 650.degree. F., the surface of the disk becomes covered with aluminum oxide. Although the oxide film is very thin, its presense interferes with the job of polishing to an even flat fine finish.
Still another problem with flatbaking at 650.degree. F. is that the disks become stuck together and are often extremely difficult to separate after they have cooled to room temperature. Yet another problem exists with the present method of flatbaking wherein disks are stacked in vertical columns. The height of the stack is limited to about twelve inches because:
there is an uneven weight between the top and bottom disks,
the stack becomes top heavy,
it is difficult to maintain alignment of the stack.
In view of these difficulties, it is an objective of this invention to provide an apparatus for flatbaking at a much reduced temperature. so that the disks will not stick together, the surfaces will not oxidize, the dwell time will be considerably shortened thereby effecting an economy of energy and oven space. It is an additional objective of this invention to provide an arrangement whereby the problems associated with limitation of vertical stacking are avoided.